Entries in Mohammed al-Qahtani
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Photo Courtesy - ABC News(NEW YORK) -- Former Defense Secretary Donald Rumsfeld declares that his biggest regret in office was not convincing President Bush to accept his resignation after the Abu Ghraib prison scandal, and said the country and the Pentagon probably "would've been better off" if he had left office in 2004.

In an exclusive interview with ABC News, Rumsfeld also said the enhanced interrogation techniques he approved for use on the so-called "20th hijacker" -- including forced stress positions and removal of "comfort items" -- produced invaluable intelligence.

But his biggest regret was staying in office during what he calls a period of "damaging distraction," as photos of abused detainees at Abu Ghraib circulated around the globe. He twice wrote letters of resignation to the president, but was convinced to stay on both times.

"That was such a stain on our country," he told ABC News. "To think that people in our custody were treated in that disgusting and perverted and ghastly way -- unacceptable way."

"There wasn't an easy target," he added. "And so I stepped up and told the president I thought I should resign. And I think probably he and the military and the Pentagon and the country would've been better off if I had."

Rumsfeld asserts that the harsh interrogation tactics he authorized for use against high-value terrorism suspects helped U.S. authorities save lives, including in the case of Mohammed al-Qahtani.

"Qahtani, who was the 20th hijacker, they say -- he gave a lot of information," Rumsfeld said. "And it was very helpful to our interrogators and to the United States government in saving lives."

He added, "And I think that the fact that we haven't had an attack in a decade is a credit to that administration, the Bush administration, and to [President Bush]."

Still, he writes in his new memoir, Known and Unknown, which is being released Tuesday, that he was "surprised and troubled" that interrogators went further than his orders allowed in some cases, particularly in interrogating Qahtani at Guantanamo Bay.

Qahtani would be subjected to sexual humiliation and long periods of sleep deprivation and isolation while in custody at Guantanamo, and his heartbeat at one point dropped to 35 beats per minute.

Rumsfeld said he only learned the details of such treatment after the fact -- and that interrogators went further than he had authorized.

"I didn't approve any of that," Rumsfeld said in the interview. "And when I found out that they had done some of those things, the people who had done things that had not been approved were dealt with. They were stopped and prosecuted."﻿